The present invention relates to a headlight of a projection type in which color fringes caused by chromatic aberration in a projection lens are suppressed, and the lens is prevented from producing a diffuse luminous intensity distribution in the horizontal direction.
A headlight currently in use is of a projection type that utilizes the same imaging principle as a projector. Such headlights have gained wide acceptance because they are small in size and yet provide good luminous intensity distribution characteristics and a broad uniform distribution of hot zones.
FIG. 18 shows schematically the composition of a projector-type headlight.
The projector-type headlight a has such a construction that beams of light issued from a light source b and which have been reflected by an elliptic reflector c are gathered at a position near the top edge of a shield plate d, which is slightly offset from the reflector c in either a forward or a backward direction, whereby a predetermined amount of light is cut off. Thereafter, the inverted image of the shield plate d is projected far forward by means of a projection lens e positioned ahead of the shield plate d. Because of this construction, the headlight a forms a cut line (cutoff line) characteristic of a low beam. The line L-L indicated by a one-long-and-one-short dashed line signifies the optical axis.
The projection lens e has a flat surface on the side closer to the light source. The other side from which light emerges is typically aspheric, with the focus being near the top edge of the shield plate d.
FIG. 19 shows the shape of the projection lens e as seen from the front. The projection lens e has rotational symmetry with respect to the optical axis L-L (normal to the plane of the paper) that passes through the intersection O of a horizontal axis RH-LH and a vertical axis UV-DV, and which extends in forward and backward directions at right angles with respect to those axes.
One of the problems known to occur in the above-described projector-type headlight is that, on account of the chromatic aberration in the projection lens e, light departing from the paraxial region is separated into spectra to produce an iridescent pattern near the cut line, thereby causing lower visibility. This phenomenon occurs for the following reason.
As shown in FIG. 18, light entering the marginal portion of the projection lens e is separated into spectra on account of the chromatic aberration in the lens. That is, the blue light l.sub.b is refracted more strongly towards the optical axis than the red light l.sub.r. As a result, color fringes will form in the vicinity of the cut line. In certain cases such as when the vehicle pitches (when its front end rises or falls), the headlight may produce color shades that occasionally can be confused with a signal light or a marker light, thus resulting in a safety hazard. In addition, the headlight can experience a color change to red or blue depending on the viewing angle, which causes a problem such as discomfort or dazzling to the driver of an oncoming vehicle or a pedestrian.
As shown by hatched areas in FIG. 19, the blue chromatic aberration is noticeable in region g closer to the lower edge of the lens, whereas the red chromatic aberration is noticeable in region f closer to the upper edge of the lens. It has been found though that the blue chromatic aberration produces a particularly great sense of "strangeness". Therefore, a need exists for improving the shape of the lower part of the lens.
A further problem is that in the case where the reflector of the projector-type headlight has rotational symmetry with respect to the optical axis, the lens is unable to produce adequate beam spread in the horizontal direction. To cope with this problem, there is a need to construct the reflector with a compound surface or the like, but this involves certain difficulties in the making of a suitable mold, and is cumbersome in the accompanying need to take into account the effect of molding precision on the performance of luminous intensity distribution.